The present invention relates to high pressure hydraulic pumps, and particularly to pumps for supplying diesel fuel at high pressure in a fuel injection system for vehicles.
Rotary hydraulic pumps for use in diesel fuel injection systems for internal combustion engines, have been well known for a number of years. Recently, desired improvements in fuel efficiency and emissions control, have led the automotive industry toward development of so-called common rail fuel injection systems, whereby a high pressure pump is utilized to establish and maintain a high fuel pressure in an accumulator in fluid communication with individual injectors. Individual injection events are controlled at the injectors for achieving combustion in the individual combustion chambers of the internal combustion engine. This is in contrast to the more common distributor type fuel injection pumps, whereby fuel pulses are distributed from within the pump, to individual distribution paths leading to a respective plurality of injectors.
Common rail pumps are expected to operate at about 20,000 psi, whereas conventional distributor pumps operate at less than about 10,000 psi. This difference accentuates certain drawbacks in conventional pumps, such as an excessive amount of fuel that experiences pressurization in connection with the pumping action, and the excessive amount of heat carried by fuel which pressure pumping, but which is not actually injected into the combustion chambers.
Unfortunately, many of the disadvantages of distributor type pumps in this regard, have been carried over into attempts to modify the distributor type pumps, for use in common rail systems. The problem of excess pumping and associated heat generation, arise especially in the so-called pump-spill, spill-pump-spill, and fill-spill techniques, as exemplified in commonly owned U.S. Pat. Nos. 5,215,449 and 5,688,110. The reluctance in setting aside such spill-type pumps, is that the fuel delivery requirements on the pump can vary considerably depending on, for example, whether the pump is starting from a cold condition, whether the pump is running at a sustained, steady state condition, and whether acceleration is required to handle an increased load. With the spill-type pumps, a quantity of fuel is delivered to the pump in an amount greater than any necessary requirement, and spill control is utilized during pumping, to try to match the quantity discharged from the pump, with the instantaneous requirements.
Other techniques attempt to match fuel quantities delivered to the pumping chamber, with the instantaneous requirements, e.g., pre-metering based on computations of pump demand by an electronic control unit (ECU). This pre-metering of the fuel quantity to be charged into the pumping chamber is typically controlled by a solenoid valve responsive to a control signal from the ECU. A major disadvantage of solenoid-implemented pre-metering, is the relatively long duration required for the metering of a useful quantity of fuel through the solenoid valve, and the difficulty to adjust the metered quantity over a wide range according to the needs of the engine. In many instances, the intake phase of pumping chamber operation with pre-metering, would not leave sufficient time to implement the pumping phase using a cam pumping rate profile shallow enough to assure quiet operation. Even with dual-rate pumping profiles, there is not enough time available during the pumping phase of a cycle, to incorporate such duality.
Another consideration which leads to significant disadvantages in the use of conventional distributor pumps for common rail injection systems, is the relatively long fuel flow paths associated with feed and discharge phases of operation.
It is, therefore, an object of the present invention to provide a high-pressure hydraulic pump which minimizes the quantity of fuel charged in the pumping chamber during the intake phase of operation, is highly energy efficient during steady state pumping operation, yet in the preferred embodiment can respond quickly to transients, such as acceleration.
It is another object, notwithstanding the nature of the manner in which fuel is fed to the pump, that the flow passages within the pump minimize dead volume and provide for highly efficient charging and discharging valve operation.
The objects set forth above are satisfied according to the present invention, by a cam having an internal actuation profile for the simultaneous inward actuation of a plurality of pumping plungers, whereby the fuel is pressurized in a substantially common, central pumping chamber, from which the high pressure fuel is discharged through an outlet located substantially on the drive shaft axis.
In one aspect, the invention has a pump housing which includes a substantially cylindrical cavity in which a stationary body portion is mounted, the body including a stationary hub portion which carries the shoe and roller assemblies radially outwardly of the plunger bores. The body includes another longitudinal, central cavity, which contains a high pressure outlet fitting. An axially slidable control valve is supported within the fitting. An inlet chamber and the inlet check valve are situated along the axis of the fitting. The inlet check valve and the discharge check portion of the control valve are both located close to the plane passing through all the pumping plungers. As a result, the flow passage between the inlet check valve and the pumping chamber, and from the pumping chamber to the discharge check valve portion, are relatively short, minimizing dead volume. Furthermore, the substantially axial flow path from the discharge check valve, along the control valve and through the fitting for discharge through a single outlet at over 100 bars, provides significant efficiencies and advantages.
The invention may also be considered as a high pressure fuel supply pump, comprising a pump housing and a pump body fixed within the housing along a body axis and including a plurality of radially oriented plunger bores, each bore having a pumping plunger disposed therein for reciprocal radial motion. An actuating assembly is disposed within the housing and around the plunger bores for producing the reciprocal motion by simultaneously driving the plungers radially inwardly during a pumping phase of the operation and simultaneously retracting the plungers radially outwardly during a charging phase of operation. A central cavity extends along the axis and intersects the pumping bores to form a pumping chamber in cooperation therewith. A feed fuel supply train includes an inlet check valve biased to open and fluidly expose the plunger bores to a supply of feed fuel at a relatively low pressure during the charging phase of operation and to seal against the supply of feed fuel during the pumping phase of operation. A high pressure outlet fitting is fixed in the central cavity and includes an internal valve cavity in fluid communication with the pumping chamber and extending along the axis. A discharge check valve is biased to seal the valve cavity from the pumping chamber while the inlet check valve opens to deliver low pressure fuel to the pumping bores during the charging phase of operation and to fluidly expose the valve cavity to the pumping chamber during the pumping phase of operation. In this manner, during the pumping phase of operation, high pressure fuel is discharged from the pumping chamber through the outlet fitting substantially along the axis.
The preferred implementation includes the steps of pre-metering successive quantities of fuel from a reservoir to a positive displacement transfer pump, then actuating the transfer pump to raise the pressure of the successive quantities of fuel by at least about 100 psi, preferably 200-300 psi. Each quantity of fuel which was pressurized in the transfer pump, is delivered to a high pressure pumping chamber defined in part by a plurality of fluidly interconnected high pressure pumping bores, so that each pumping bore receives a certain, i.e., predetermined, charge of fuel within a first time interval. A plurality of plungers in the respective pumping bores are then simultaneously actuated to increase the pressure in the pumping chamber to the desired high pressure, preferably at least about 15,000 psi, within a second time interval, and discharge the quantity of fuel through a high pressure discharge valve. The second time interval is of longer duration than the first time interval. The first time interval can be relatively short, because the pumping chamber is charged by the transfer pump at a pressure of at least about 100 psi, which is considerably higher than the conventional charging pressure. As a result, the necessary quantity of fuel can be delivered to the pumping chamber in a relatively short time period. Therefore, each pumping plunger can be actuated by a dual rate cam profile over a relatively long time period such that at steady state the actuation occurs only along a relatively shallow slope of the cam profile, whereas when acceleration is required, the actuation can occur more quickly, along a steeper profile, before continuing along the relatively shallow profile.